In systems for transmitting and receiving data in the form of analog signals over telephone lines, the data are represented by modulation states of a carrier signal. The modulation states are taken from a finite set of discrete amplitude and/or phase values. In general, the carrier signal is modulated by a modulator at the transmission end and demodulated by a demodulator at the reception end, the modulator/demodulator assembly being called a "modem".
Some transmission systems operate in "continuous carrier" mode, i.e. the carrier signal is transmitted over the telephone line even when there is no data transmitted.
But in some other transmission systems, such as multipoint communication systems, the terminals are requested to take turns in transmitting their data to the control system. Then, when a terminal ceases transmitting, it is necessary to interrupt the carrier as well in order to allow the control system to give another terminal the opportunity to transmit. Since it is impossible to detect the exact moment when transmission ceases, the system has to be provided with a device for detecting the end of transmission and allowing the time interval between the exact end of transmission and the detection of said end of transmission, to be reduced to a minimum.
The easiest way to detect the end of transmission consists in adding a series of "zeroes" to the data. After reception of a certain number of "zeroes" by the receiving device, the detection logic unit determines and signals that transmission is terminated. Unfortunately, some line disturbances are sufficient to prevent the end of transmission from being detected.
Another method consists in using a protocol indicating the end of transmission by a particular sequence of bits. This method has several drawbacks. First of all, unlike the sequence of "zeroes", it does not allow a line cut-off to be detected since in this case, the detection logic unit does not receive said particular sequence of bits. Further, this method excludes some combinations of bits used as said particular sequence of bits and consequently, requires a system with a larger processing capacity. In addition, if an efficient noise protection is required, a particular sequence long enough is to be used, which therefore increases the time needed to detect said end of transmission.
In order to overcome the above-indicated drawbacks and to be also able to detect an end of transmission caused by a line cut-off, a method consisting in detecting the energy drop of the received signal has been used. Said received signal is sampled, the energy is measured for each sample and a comparison with a predetermined threshold is performed.
The latter method can be advantageously applied in the transmission systems using phase modulation. But in the present high speed systems using modulation states taken from constellations with a plurality of amplitude levels, the end of transmission can happen either on a high or low level of amplitude. The signal energy will take more or less time to go below the predetermined energy threshold according to whether the end of transmission happens on a high or low level of amplitude. Therefore, it is impossible to have a perfect knowledge of the time interval between the end of transmission and the detection of said end of transmission.